Calibration systems and methods

ABSTRACT

A calibration system for calibrating an antenna array comprising a plurality of antenna elements comprises a signal generator for generating a predetermined test signal and, the signal generator being couplable to the antenna elements for providing the test signal to the antenna elements, a number of probes for measuring at least one physical parameter, which is influenced by emissions of the antenna elements, and providing respective measurement signals, and a position determination unit for determining based on the measurement signals the positions of the antenna elements as calibrated positions.

PRIORITY CLAIM

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/368,362, filed Jul. 29, 2016, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The invention relates to calibration systems and methods for calibratingantenna arrays and RF devices.

BACKGROUND

Although applicable to any system that uses wireless signals, thepresent invention will be described in combination with testing ofwireless communication devices.

Modern wireless communication devices use radio frequency signal totransmit data and or speech. Manufacturers of such communication devicesalways try to improve the efficiency of the communication devices and atthe same time have to fulfil legal or regulatory regulations.

Therefore, extensive testing of such communication devices is performedduring development, production and after production. Such testing servesquality assurance and compliance tests.

SUMMARY

There is a need for improved testing of wireless devices.

In a first aspect, the invention provides a calibration system forcalibrating an antenna array comprising a plurality of antenna elements,i.e. two or more antenna elements. The system comprises a signalgenerator for generating a predetermined test signal and, the signalgenerator being configured to provide the test signal to the antennaelements, a number of probes, i.e. one or more probes, for measuring atleast one physical parameter, which is influenced by emissions orreceptions of the test signal by the antenna elements, and providingrespective measurement signals, and a position determination unit fordetermining based on the measurement signals the positions of theantenna elements as calibrated positions.

In modern communication systems, like e.g. smartphone or other mobileequipment, especially so called 5G equipment, antenna arrays can be usedto provide compact antenna systems. Such antenna arrays can perform beamforming and are therefore advantageous for improving the signal qualityor signal strength in mobile data communications.

Small deviations of the positions of the antenna elements by e.g. just0.2 mm would already comprise a deviation of 20% or more of thewavelength especially at millimeter wave frequencies. Such deviationscan therefore strongly influence the quality of the beamforming in suchdevices.

Especially with increasing frequencies and therefore smaller wavelengthsin the range of 1 mm or less, it is therefore important to exactly knowthe positions of the single antenna elements of such an antenna array.

Since in most applications the mobile device will have a cover or theantenna array will comprise a protective sheet or layer, it is difficultor impossible to determine the positions optically.

The present invention therefore provides a test signal to the antennaarray, which drives the antenna elements to emit an electromagneticsignal, which is then measured as the physical variable by the probes.Or the present invention provides a test signal to the probes to emit anelectromagnetic signal, which is received by the antenna array, i.e. theantenna elements, and which is then measured as the physical variable.This can e.g. be performed by a transceiver in a device which carriesthe antenna array or by a dedicated measuring device, which is coupledto the antenna array for the measurement.

Based on the respective measurement signals the position determinationunit will determine positions of the single antenna elements. Theposition determination unit can e.g. determine the relative positions ofthe antenna elements, i.e. the positions of the antenna elementsrelative to an origin of a predetermined coordinate system.

The determined positions can then just exemplarily be provided to thedevice under test, DUT, i.e. the respective smartphone or the like,which can then calibrate its transceiver accordingly. The determinedpositions can also be used by test equipment e.g. in an end of lineverification after production of the respective device. To support suchend of line tests, the calibration system can also store a serial numberof the single DUTs in order to match a set of positions to therespective DUT.

In one embodiment, the calibration system can further comprise a motionactuator for moving relative to each other the probes and the antennaarray, and a recorder for recording the measurement signals togetherwith the respective positions of the probes. Moving the probes and theantenna array relative to each other means that either the probes can bemoved, the antenna array can be moved or both can be moved.

By moving the probes and the antenna elements relatively to each otherdetailed measurements of the physical variable can be performed, whichat least for the most part or even completely cover the space in frontof the antenna elements.

In one embodiment, the position determination unit can identify thepositions of the antenna elements based on maxima of the measurementsignals and the positions of the respective maxima. By measuring thephysical variable in front of the antenna elements, the probes willacquire higher values for the measurement signals when the respectiveprobe is directly in front of the respective antenna element. Thefarther away the probe moves from the antenna element, the lower will bethe acquired value of the measurement signal. Therefore, using themaxima of the measurement signal to identify the positions of theantenna elements provides a simple method for identifying the positions.

It is understood, that the distance of the points at which the probesmeasure the physical variable is chosen according to the desiredresolution of the position determination. That means that the distanceis at maximum as large as the desired resolution, i.e. between 0.01 mmand 1.0 mm, e.g. 0.05 mm or 0.1 mm.

In one embodiment, the position determination unit can identify thepositions of the antenna elements based on positions of the shifts, atwhich 1D or 2D auto-correlations of a magnitude pattern of the measuredsignals provide maxima.

In one embodiment, the position determination unit can identify thepositions of the antenna elements based on a two dimensional Fourierseries or Fourier Transform together with a measurement heatmap of themeasured signals.

In one embodiment, the motion actuator can move relative to each otherthe probes and the antenna array such that the probes move in a firstplane that is parallel to a second plane, in which the antenna elementslie. The distance between the planes is ideally chosen small enough fordifferentiating the signals of the single antenna elements in theresulting measurement signal. The relative movement can especially beperformed such the probes cover the antenna array sufficiently tomeasure the physical parameter for every single antenna element. Theresult can e.g. be a two dimensional matrix, where every matrix elementidentifies a probe position, while the value of the respective matrixelement represents the value of the respective measurement signal. Sucha matrix can also be called heat map. This term becomes obvious if thematrix is displayed as a two-dimensional diagram, in which higher valuesare displayed with increasingly more intense red colors, while lowervalues can e.g. be displayed with green or blue colors.

In one embodiment, the motion actuator can move relatively to each otherthe probes and the antenna array such that the probes move in aplurality of third planes that are parallel to the first plane and eachdistanced apart from the prior plane by a predetermined distance. Themotion actuator will therefore move the probes in a three dimensionalspace, which lies directly over or in front of the antenna elements. Themeasurements performed with such a movement, will result in a threedimensional matrix or heat map, which comprises cone or funnel shapedstructures, corresponding to the positions of the antenna elements.

The motion actuator can e.g. comprise electric motors and respectivemechanical guides, fixtures and the like for affixing the probes and orthe DUT. The motion actuator can e.g. comprise a portal arrangement ase.g. used in milling machines or 3D printer. An alternate arrangementcould e.g. be a delta printer like arrangement. Alternatively, anX-Y-moving plate can be provided. It is understood that thesearrangements are mere examples and that any other arrangement can beused.

In one embodiment, the motion actuator can move the probes to estimatedstart positions, which are estimated to lie in front of respectiveantenna elements and further moves the probes in a pre-defined zoneuntil a maximum measurement value is identified. This means that themotion actuator can perform a kind of spatially limited search for themaximum at and around the estimated positions of the single antennaelements. The pre-defined zone can e.g. be as large as the antennaelements plus the position tolerances of the antenna elements. It isunderstood that any other size can be chosen for the pre-defined zone.

In one embodiment, the signal generator can provide the test signal toall antenna elements at the same time. This means that all antennaelements radiate a respective signal at the same time. If more than oneprobe is used, a quick measurement can be performed, if the probes aree.g. moved line-wise in front of the antenna array and all antennaelements radiate a signal.

In one embodiment, the signal generator can consecutively provide thetest signal to the antenna elements one by one, or provide the testsignal to pairs of the antenna elements or to at least three of theantenna elements at the same time. During the measurement, only oneantenna element will therefore be active at a given time. The respectivemeasurement signals will therefore not be influenced by neighboringantenna elements and the distinction between single antenna elementswill be improved.

It is understood, that the signal generator can also drive the singleantenna elements in any adequate pattern to mitigate the mutualinfluence between neighboring antenna elements and at the same timeimprove the measurement speed by measuring more than one antenna elementat a time.

In one embodiment, the signal generator can generate the test signalcomprising a radio frequency, RF, signal of a predetermined frequency.

The signal generator in this case can directly drive the single antennaelements. This is especially useful for DUTs, which comprise a testconnector to the antenna elements.

In one embodiment, the signal generator can generate the test signalcomprising a digital command signal for a transceiver of the antennaarray, which commands the transceiver to drive the single antennaelements with a radio frequency, RF, signal of a predeterminedfrequency. The signal generator in this case can instruct DUTs, which donot comprise a dedicated connector to the antenna elements, to generatethe test signal as needed.

In one embodiment, the probes can comprise a measurement element formeasuring the value of the physical parameter and/or electromagneticsignals. Further, the probes can comprise a transmitting element fortransmitting the predetermined test signal and/or electromagneticsignals. Also a single element, like e.g. an antenna, can be provided asreceiving and transmitting element.

In one embodiment, the measurement element can comprise an antenna.

In one embodiment, the antenna can be adapted to the frequency of thetest signal. This means that the antenna's frequency range or band istuned to the test signal.

In one embodiment, the measurement signal can comprise a voltage and/ora current and/or a power and/or a phase of the measured physicalparameter.

In one embodiment, the physical parameter can comprise an electric fieldand/or a magnetic field and/or an electromagnetic field.

It is understood that the above embodiments of the first aspect canmutatis mutandis be implemented in a respective calibration method. Itis further understood, that the elements of the calibration system canbe implemented in hardware, software, hardware description, e.g. in aCPLD or FPGA, or any combination of the above. Further, the calibrationmethod can also be implemented at least partially in a computer, i.e. asa computer implemented method.

In a second aspect a calibration system for calibrating a radiofrequency, RF, device comprising a plurality of signal paths, eachsignal path comprising at least an amplifier and an antenna element, isprovided. The system comprises a measurement system for driving thesignal paths with a predetermined test signal and measuring an output ofthe signal paths in response to the test signal, a determination modulefor determining a first signal path, of which the antenna elementprovides the lowest output of all antenna elements, and a correctionfactor calculator for calculating based on output of the first signalpath a correction factor for the further signal paths such that with theapplied correction factor the output of all signal paths is equal withina predetermined acceptance interval.

Just exemplarily the measurement system can be a or part of acalibration system according to claim 1 or any one of its dependentclaims. However, the measurement system can be any system that iscapable of driving the signal paths and measuring the outputsaccordingly. Usually it will be necessary for the measurement system toknow the exact positions of the antenna elements. The positions of thesingle antenna elements can e.g. be determined with a calibration systemaccording to claim 1 or any one of its dependent claims. However, thepositions of the single antenna elements can also be provided by anyother means.

With the predetermined test signal, all the signal paths should providethe same output, i.e. a signal with the same output power. However, dueto tolerances in the single elements of the signal paths, the singlesignal paths will provide different outputs. Especially when the antennaarray is used for beamforming, such deviations in the output power ofthe single signal paths can deteriorate the quality of the beam-formedsignal.

The calibration system mitigates these negative effects by providing acalibration of the single signal paths, such that with the same nominalinput signal all signal paths provide the same output. As a basis forthe calibration, the signal path with the lowest output is used and thecorrection factors are calculated in relation to this lowest output.

When these correction factors are applied to the signal paths, with thesame nominal input signal the output will be equal within an acceptablerange or acceptance interval.

In one embodiment, the measurement system can measure a physicalparameter, which is influenced by emissions of the antenna elements inresponse to the test signal, in front of the antenna elements as theoutput and provides respective measurement values. The measurement datacan e.g. be provided in the form of a two-dimensional orthree-dimensional matrix, where every matrix element identifies aposition in front of the antenna array, while the value of therespective matrix element represents the measured value of the physicalparameter measured at the respective position.

In one embodiment, the determination module can comprise anidentification unit, for identifying the values of the measurement data,which represent measurements in front of the positions of the antennaelements. That means the values of the measurement data, which representthe single signal paths.

The determination module can e.g. analyze the output for the singlesignal paths and determine the respective maximum values.

In one embodiment, the determination module can comprise a comparator,which is configured to compare the values of the measurement data, todetermine the first signal path.

In one embodiment, the correction factor calculator can comprise adivider for dividing the lowest output value, which represents the firstsignal path, by the output value, which represents a respective otherone of the signal paths, for calculating the correction factor for saidother signal path.

In one embodiment, the calibration system can further comprise averification unit for verifying that all signal paths provide an output,which is larger than a predetermined minimum output. With thepredetermined test signal, every signal path should provide—withincertain tolerances—a similar output. Is a single signal paths provides asignificantly lower output, that signal path may be defective. Theverification unit therefore serves to verify correct functionality ofthe signal paths and to identify faulty RF devices. The required minimumoutput can e.g. be set by a user.

In one embodiment, the test signal can drive the signal paths to apredetermined nominal power level. The nominal power level regarding thepresent patent application is a power level, which is set by the DUT ascommanded for all signal paths. That means that in terms of the DUT theoutput of all signal paths should be the same. However, as alreadyexplained above, tolerances can lead to different outputs of thedifferent signal paths. If all the signal paths are driven to the samenominal power level, it is easy to identify deviations of the singlesignal paths.

In one embodiment, the test signal can set the amplifiers of the signalpaths all to the same nominal gain value.

In one embodiment, the test signal can drive the signal paths to apredetermined maximum power level.

If the signal paths are driven to their respective maximum power level,the weakest signal path can easily be identified.

In one embodiment, the test signal can comprise a radio frequency, RF,signal of a predetermined frequency. This is especially useful for RFdevices, which comprise a test connector to the antenna elements.

In one embodiment, the test signal can comprise a digital command signalfor a transceiver of the RF device, which commands the transceiver todrive the single signal paths with a radio frequency, RF, signal of apredetermined frequency and/or to set the amplifiers of the singlesignal paths to a predetermined nominal gain factors. The signalgenerator in this case can instruct RF devices, which do not comprise adedicated connector to the antenna elements, to generate the test signalas needed.

In one embodiment, the correction factors can be provided as gainfactors for the amplifiers of the respective signal paths. This type ofcorrection factors can be used to directly set the gain in the singlesignal paths without any further signal processing.

In one embodiment, the correction factors can be provided as digitalvalues to a signal-processing unit of the RF device, which drives thesignal paths. If the correction factors are provided as digital valuesto the signal-processing unit, the signal-processing unit can decide howto use the correction factors. The signal-processing unit can set thegain factors of the single amplifiers in the signal paths. However, asan alternative, the signal-processing unit can also modify, i.e. amplifyor attenuate, the signals, which are provided to the single signalpaths. This allows using fixed gain amplifiers and at the same time,providing calibrated output signals.

In one embodiment, the measurement system can transmit the test signalto the antenna elements and receives from the RF device the measuredoutput of the signal paths. When the measurement system drives thesignal paths from within the RF device, the calibration system cancalibrate the transmitting signal paths of the RF device. However, itthe measurement system sends the test signal to the antenna elementswirelessly via e.g. probe antennas, the RF device can internally measurethe output of the receiving signal paths and provide the calibrationsystem with the respective output.

It is understood that the above embodiments of the second aspect canmutatis mutandis be implemented in a respective calibration method. Itis further understood, that the elements of the calibration system canbe implemented in hardware, software, hardware description, e.g. in aCPLD or FPGA, or any combination of the above. Further, the calibrationmethod can also be implemented at least partially in a computer, i.e. asa computer implemented method.

In a third aspect, the present invention provides a calibration systemfor calibrating a radio frequency, RF, device comprising a plurality ofsignal paths, each signal path comprising at least an amplifier and anantenna element. The system comprises a signal generator for driving thesignal paths with a predetermined test signal, at least two probes formeasuring the output of the signal paths in reaction to the test signal,and a correction factor calculator for calculating respective correctionfactors based on differences in at least one characteristic of themeasured outputs of the signal paths.

In modern communication systems, like e.g. smartphone or other mobileequipment, especially so called 5G equipment, antenna arrays can be usedto provide compact antenna systems. Such antenna arrays can perform beamforming and are therefore advantageous for improving the signal qualityor signal strength in mobile data communications.

However, the single signal paths in such devices will each comprisetolerances, which will slightly modify the signals while beingpropagated to the antenna elements. If the signals are then transmittedby the antenna elements e.g. with a phase or a frequency that deviatesfrom the intended signal, the transmitting capabilities and especiallythe beamforming capabilities may deteriorate.

With the predetermined test signal being provided to all signal paths,all the signal paths should provide the same output, i.e. a signal withthe same phase, the same amplitude, the same frequency and the sametiming. However, as already indicated the single elements of the signalpaths, in transmitting direction as well as in receiving direction, willcomprise tolerances. These tolerances are introduced into the elementsduring production due to inevitable inaccuracies and size variations.Even though these tolerances may be minute, they will still influencethe signal propagation in the respective signal path. Therefore, thesingle signal paths will provide different outputs, when provided withthe same input signal, i.e. the test signal.

Especially, when the antenna array is used for beamforming, suchdeviations in the output power and phase of the single signal paths candeteriorate the quality of the beam-formed signal. The calibrationsystem mitigates these negative effects by providing a calibration ofthe single signal paths. Calibration in this context refers to amodification or configuration of elements of the signal paths or thesignal generation, such that the differences caused by the tolerances inthe signal paths are balanced.

The use of at least two probes, i.e. antennas, allows analyzing thesingle measured outputs and especially the differences between themeasured outputs in real time and in more depth than if the outputswhere analyzed separately.

In one embodiment, the characteristics of the measured outputs cancomprise a phase and/or an amplitude and/or a frequency and/or a timingof the outputs.

In one embodiment, the correction factor calculator can calculate thecorrection factor based on a difference in phase of the measuredoutputs.

In one embodiment, the correction factor calculator can calculate thecorrection factor based on a difference in amplitude of the measuredoutputs.

In one embodiment, the correction factor calculator can calculate thecorrection factor based on a difference in frequency and/or timing ofthe measured outputs.

In one embodiment, the correction factor calculator can provide thecorrection factors directly to elements of the respective signal paths.This type of correction factors can be used to directly set e.g. thegain, a phase shift and/or a frequency in the single signal pathswithout any further signal processing. The signal paths can e.g.comprise configurable phase shifters or the like, which can beconfigured. It is understood, that directly configured refers to settingparameters of elements of the signal chain as opposed to performsignal-processing calculations. This means that the correction factorscan also be provided to any kind of processing unit in the RF device,which configures the correction factors in the respective elements.

In one embodiment, the correction factor calculator can provide thecorrection factors as digital values to a signal-processing unit of theRF device, which drives the signal paths. The signal-processing unitopposed to the above can modify, i.e. amplify or attenuate, the signals,which are provided to the single signal paths via signal processingcalculations. This allows using fixed gain amplifiers or other fixedelements in the signal paths and at the same time providing calibratedoutput signals.

In one embodiment, the signal generator can generate the test signalcomprising a radio frequency, RF, signal. The RF signal can have apredetermined frequency or timing, a predetermined phase and/or apredetermined amplitude. This kind of test signal is especially usefulfor RF devices, which comprise a test connector to the antenna elements.

In one embodiment, the signal generator can provide the RF signal tointernal connectors of the RF device for driving the signal paths withthe RF signal. The signal generator in this case can instruct RFdevices, which do not comprise a dedicated connector to the antennaelements, to generate the test signal as needed.

In one embodiment, the signal generator can provide the RF signal to theprobes for transmitting the test signal to the respective antennaelements of the respective signal paths. If the test signal istransmitted wirelessly to the antenna elements by the probes, thereceiving signal path of the respective antenna element can be analyzed.The output can e.g. be provided to the calibration system by the RFdevice via digital interface.

In one embodiment, the signal generator can provide a first sub-signalof the RF signal to the probes for transmitting the test signal to therespective antenna elements of the respective signal paths, and whereinthe signal generator provides a second sub-signal of the RF signal tothe RF device for driving the signal paths with the RF signal. The firstand the second sub-signals e.g. both comprise different frequencies. Asan alternative, the first and the second sub-signals can be interleavedin a timely fashion. This allows testing the sending and the receivingparts of the signal paths at the same time.

In one embodiment, the signal generator can generate the test signalcomprising a digital command signal for a transceiver of the RF device,which commands the transceiver to drive the single signal paths with aradio frequency, RF, signal of a predetermined frequency. The signalgenerator in this case can instruct RF devices, which do not comprise adedicated connector to the antenna elements, to generate the test signalas needed.

In one embodiment, the probes can be spaced apart from each other by apredetermined distance, which defines an exclusion zone. The distancebetween the probes makes sure that no mutual influences falsify themeasurements.

In one embodiment, the predetermined distance can define a distancebetween the apertures of two probes.

In one embodiment, the predetermined distance can be at least 0.4 timesthe wavelength of the test signal, especially of the test signal in thematerial that comprises more than 50% of the space between the probes.

It is understood that the above embodiments of the third aspect canmutatis mutandis be implemented in a respective calibration method. Itis further understood, that the elements of the calibration system canbe implemented in hardware, software, hardware description, e.g. in aCPLD or FPGA, or any combination of the above. Further, the calibrationmethod can also be implemented at least partially in a computer, i.e. asa computer implemented method.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention andadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings. The invention isexplained in more detail below using exemplary embodiments, which arespecified in the schematic figures of the drawings, in which:

FIG. 1 shows a diagram of an embodiment of an antenna array;

FIG. 2 shows a block diagram of an embodiment of a calibration systemaccording to the present invention;

FIG. 3 shows a diagram of an embodiment of measurement signals accordingto an embodiment of the present invention;

FIG. 4 shows a block diagram of another embodiment of a calibrationsystem according to the present invention;

FIG. 5 shows flow diagram of an embodiment of a method according to thepresent invention;

FIG. 6 shows a block diagram of another embodiment of a calibrationsystem according to the present invention;

FIG. 7 shows a diagram of an embodiment of measurement signals accordingto an embodiment of the present invention;

FIG. 8 shows a block diagram of another embodiment of a calibrationsystem according to the present invention;

FIG. 9 shows flow diagram of another embodiment of a method according tothe present invention;

FIG. 10 shows a block diagram of another embodiment of a calibrationsystem according to the present invention;

FIG. 11 shows a block diagram of another embodiment of a calibrationsystem according to the present invention; and

FIG. 12 shows flow diagram of another embodiment of a method accordingto the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an embodiment of an antenna array 1,which comprises a number of antenna elements 2, 3. For sake of clarityonly the first and the last antenna elements 2, 3 have been providedwith a reference sign.

The antenna array 1 comprises four columns and six lines of antennaelements 2, 3, i.e. a total of 24 antenna elements. The single antennaelements 2, 3 are equidistant to each other column-wise and row-wise.This antenna array 1 is just an exemplary antenna element. Other antennaelements can have any number of antenna elements spaced apparat at anydistances.

FIG. 2 shows a block diagram of an embodiment of a calibration system 4according to the first aspect of the present invention. The calibrationsystem 4 comprises a signal generator 5, which is coupled to the singleantenna elements 2, 3 via signal lines, which are shown in FIG. 2 as asignal bus 6. The signal bus 6 carries the test signal 7, which thesignal generator 5 generates to the single antenna elements 2, 3. InFIG. 2, the signal generator 5 is connected to the antenna elements 2, 3via a single signal bus 6. However, this bus is just exemplarily drawnas a single line. In fact, the signal bus 6 can comprise any number ofdata lines. The signal bus 6 can especially comprise a dedicated signalline to each one of the antenna elements 2, 3. This allows the signalgenerator 5 to drive specific single antenna elements 2, 3 or specificgroups of antenna elements 2, 3. In another embodiment, the signalgenerator 5 can be indirectly coupled to the antenna elements 2, 3 via asignal-processing device of the antenna array 1 or the device, whichcarries the antenna array 2, e.g. a smartphone.

The signal generator 5 may comprise any means, which are capable ofgenerating the respective signal. Such means may e.g. comprise anoscillator coupled to further signal generation devices, like e.g. e PLLor the like. If the signal generator 5 is coupled to a signal processingdevice as explained above, the signal generator 5 can comprise means forgenerating a digital data signal, which comprises information about thetest signal 7 such that the signal processing device can generate thetest signal 7 for the respective antenna elements 2, 3.

The calibration system 4 further comprises a probe 8, which can be movedat least in two axis, i.e. a plane. This plane can especially beparallel to the plane of the antenna array 1. The distance between theplanes must be small enough for the signals of the single antennaelements 2, 3 to be distinguishable. If the probe moves to far away fromthe antenna elements 2, 3 the signals of different antenna elements 2, 3will blend into each other and no exact measurement will be possible. Itis understood that any number of probes 8 can be used, which can bepositioned either statically to each other or be movable relative toeach other, i.e. they can be moved separately. It is obvious thatincreasing the number of probes 8 will also increase the measurementspeed.

The probe 8 can comprise in one embodiment an antenna (not explicitlyshown, see FIG. 3) that is tuned to the frequency of the test signal,e.g. 28 GHz. The probe 8 can provide the measurement signal 9 comprisinga voltage, a current and/or a power.

The probe 8 and the signal generator 5 are both coupled in bidirectionalcommunication to the position determination unit 10, which in theembodiment of FIG. 2 further incorporates a central control unit of thecalibration system 4. That means that the position determination unit 10can control the signal generator 5 and the probe 8. For example, theposition determination unit 10 start the signal generator 5 or move theprobe 8 to a specific position.

The probe 8 will deliver the measurement signals 9 to the positiondetermination unit 10. The position determination unit 10 will analyzethis measurement signals to determine the exact positions of the antennaelements 2, 3 and output said positions as calibrated positions 11.

The calibrated positions can e.g. be provided relative to the origin 15of a predefined coordinate system (see FIG. 3). The coordinate systemcan also be known to the device, which carries the antenna array 1. Thisallows the device to use the calibrated positions 11 to internallycalibrate or adjust the stored positions for the single antenna elements2, 3.

FIG. 3 shows a diagram of an embodiment of measurement signals 9, whichare shown as two-dimensional map. The two dimensional map is a kind ofcontour graph, where the intensity of the measurement signal isrepresented by the distance of the single contour lines. That means thatthe measured intensity is higher if the distance of the contour lines issmaller. The diagram comprises a local maximum 16, 17 for each one ofthe antenna elements 2, 3, since at these positions the maximum powercan be measured by the probe 8.

As can be seen, in the diagram the origin 15 of a coordinate system isshown. The measurement values 9 of the probe 8 can all be referenced tothe position of the probe 8 relative to the origin 15. This allowseasily determining the positions of the maxima 16, 17 in thetwo-dimensional map relative to the origin 15, i.e. in the respectivecoordinate system.

FIG. 4 shows a block diagram of another embodiment of a calibrationsystem 24, which is based on the calibration system 4 of FIG. 2. Similarelements are provided with similar reference signs as in FIG. 2 butincreased by 20.

The calibration system 24 further comprises a recorder 32, which cancomprise signal converters, like e.g. analog-to-digital converters, anda memory that stores the measurement values 29 and the respectivepositions for later processing by the position determination unit 30.

The calibration system 24 also comprises a motion actuator 33 for movingthe probe 28. Although not explicitly shown, the motion actuator 33 cane.g. be controller by the position determination unit 30. Finally, theprobe 8 comprises an antenna 34 as measurement element.

FIG. 5 shows flow diagram of an embodiment of a method for calibratingan antenna array 1 comprising a plurality of antenna elements.

The method comprises generating, S1, a predetermined test signal 7, 27and providing the test signal 7, 27 to the antenna elements 2, 3. Thetest signal 7, 27 can e.g. be provided to all antenna elements 2, 3 atthe same time. As an alternative the test signal 7, 27 can be providedconsecutively to the antenna elements 2, 3 one by one or group by group.The test signal 7, 27 can comprising a radio frequency, RF, signal of apredetermined frequency, which can be directly fed into the antennaelements 2, 3.

Alternatively, the test signal 2, 27 can comprise a digital commandsignal for a transceiver of the antenna array 1, which commands thetransceiver to drive the single antenna elements 2, 3 with a radiofrequency, RF, signal of a predetermined frequency.

Further, at least one physical parameter is measured, S2, with a numberof probes 8, 28, which is influenced by emissions of the antennaelements 2, 3. Respective measurement signals 9, 29 are then analyzed todetermine, S3, the positions of the antenna elements 2, 3 as calibratedpositions 11, 31. When determining the positions of the antenna elements2, 3, these can be determined based on maxima of the measurement signals9, 29 and the positions of the respective maxima.

The method can therefore further comprise moving relatively to eachother the probes 8, 28 and the antenna array 1, and recording themeasurement signals 9, 29 together with the respective positions of theprobes 8, 28. The probes 8, 28 can e.g. be moved in a first plane thatis parallel to a second plane, in which the antenna elements 2, 3 lie.That means that the probes 8, 28 are moved in front of the antenna array1 and perform a kind of scan of the surface of the antenna array 1.

The probes 8, 28 can also be moved in a plurality of third planes thatare parallel to the first plane and each distanced apart from the priorplane by a predetermined distance. This means that a three-dimensionalscan is performed in various planes or layers.

Instead of or as addition to recording the measurement values 9, 29, theprobes 8, 28 can be moved to estimated start positions, which areestimated to lie in front of respective antenna elements 2, 3 prior tostarting the analysis. The probes 8, 28 can then be moved in apre-defined zone until a maximum measurement value 9, 29 is identified.

The probes 8, 28 can e.g. comprise a measurement element 34, like e.g.an antenna, for measuring the value of the physical parameter. Such anantenna 34 can be adapted to the frequency of the test signal 7, 27.

FIG. 6 shows a block diagram of an embodiment of a calibration system100 for a RF device 150.

The RF device comprises three of signal paths 151, 152, 153, whereinthis number is just exemplary and more or less signal paths are possible(hinted at by three dots). Every signal path 151, 152, 153 in thisexample comprises a transmit signal chain consisting of an amplifier155, an optional phase shifter 156, and an antenna element 158. The sameantenna element 158 also serves the receive signal chain, whichcomprises the antenna element 158, an optional phase shifter or filter159, and an amplifier 160. The signals of the transmit and the receivesignal chains are separated by a circulator 157. For sake of simplicity,only the elements of the first signal path 151 are provided withreference signs. The signal paths 151, 152, 153 are connected to atransceiver 154 of the RF device 150, which drives the transmit signalchains and receives signals from the receive signal chains. It isunderstood, that this arrangement of the RF device 150 is just exemplaryand serves to explain the present invention. However, the presentinvention can be used with any other RF device.

The calibration system 100 comprises a measurement system 101, whichserves to measure the signals emitted by the antenna elements 158 or totransmit test signals 107 to the antenna elements 158. If the signalsemitted by the antenna elements 158 are measured, e.g. a probe 102 ofthe measurement system 101 can detect the signals, while a signalgenerator 103 generated a test signal 107 and transmits this to thetransceiver 154 for driving the signal paths 151, 152, 153.

In this case, the test signal 107 can comprise a digital command signalfor the transceiver 154 of the RF device 150, which commands thetransceiver 154 to drive the single signal paths 151, 152, 153 with aradio frequency, RF, signal of a predetermined frequency and/or to setthe amplifiers of the single signal paths 151, 152, 153 to apredetermined nominal, e.g. the maximum, gain factors.

The probe 102 can however also be used to transmit signals to theantenna elements 158. In such a mode, the feedback is provided from thetransceiver 154 to the measurement system 101. The test signal in thiscase can e.g. be a RF signal.

The measurement system 101 can provide a heat map or two-dimensionaldiagram as shown in FIG. 3. Therefore, one possible measurement system101 is the measurement system of FIGS. 2 and 4. Based on the output ofthe measurement system 101, the determination module 104 will determinethe antenna element 158, which provides the lowest output. The magnitudeof this output will then serve the correction factor calculator 105 tocalculate correction factors 106 for the other signal paths 151, 152,153. When applied to the signal paths 151, 152, 153 the correctionfactors 106 will equate the outputs of the signal paths 151, 152, 153,at least regarding output signal strength.

FIG. 7 shows a diagram of the output of the measurement system 101. Thelower part of the diagram is a cut through the diagram of FIG. 3 at thecut line A. It can be seen, that for every circle in the diagram of FIG.3, i.e. every antenna element, there is a local maximum in the diagramof FIG. 7. The abscissa of diagram shows the location in X direction,the ordinate of the diagram shows the power (or any value that refers tothe power of the respective signal, e.g. a voltage, a current or thelike.

Further, there are two lines drawn in the diagram, a threshold line thrand a minimum line min. The threshold line thr defines a minimum value,which all antenna elements or signal paths 151, 152, 153 have tosurpass. If any one of the signal paths 151, 152, 153 does not surpassthis threshold thr, the RF device is marked as defective.

The minimum line min refers to the power level of the signal path 151,152, 153 that provides the lowest power level. That means that thisminimum power level min is the basis for later calculating thecorrection factors 106, 126.

FIG. 8 shows a block diagram of another embodiment of a calibrationsystem 120, which is based on the calibration system 100 of FIG. 6.Similar elements are provided with similar reference signs as in FIG. 6but increased by 20.

In the calibration system 120, the determination module 124 comprises afunction 127 to search all local maxima as shown in FIG. 7 and afunction 128 to compare the local maxima to find the maximum with thelowest value. This value is the provided to the correction factorcalculator 125, which comprises another function 129 to calculate basedon the magnitude of the minimal maximum the correction factors 126 forall signal paths 151, 152, 153.

The calibration system 120 further comprises a verification unit 130with a memory 131 and a function 132 that serves for verifying if allthe signal paths 151, 152, 153 provide signals with at least the minimumpower min. The value of min can e.g. be set by a user of the calibrationsystem 120. If any one of the signal paths 151, 152, 153 provides asignal lower than the minimum power min, the verification unit 130 willoutput a warning signal 133.

It is understood that the calibration systems 100, 120 can beimplemented in hardware, software or any combination therefore. Parts ofthe calibration systems 100, 120 can e.g. also be implemented in aconfigurable logic element, like e.g. a CPLD or FPGA.

FIG. 9 shows flow diagram of an embodiment of a method for calibrating aradio frequency, RF, device 150 comprising a plurality of signal paths151, 152, 153. Each signal path 151, 152, 153 comprises at least anamplifier 155 and an antenna element 158.

The method comprises driving S101 the signal paths 151, 152, 153 with apredetermined test signal 107, 134 and measuring an output of the signalpaths 151, 152, 153 in response to the test signal 107, 134. Whenmeasuring a physical parameter can be measured, which is influenced byemissions of the antenna elements 158 in response to the test signal107, 134, in front of the antenna elements 158 as the output. The testsignal 107, 134 can drive the signal paths 151, 152, 153 to apredetermined nominal, especially the maximum, gain level.

The test signal 107, 134 can comprise a radio frequency, RF, signal of apredetermined frequency. As an alternative, the test signal 107, 134 cancomprise a digital command signal for the transceiver 154 of the RFdevice 150, which commands the transceiver 154 to drive the singlesignal paths 151, 152, 153 with a radio frequency, RF, signal of apredetermined frequency and/or to set the amplifiers 155 of the singlesignal paths 151, 152, 153 to a predetermined nominal gain factor.

Based on the measured output, a first signal path 151, 152, 153, ofwhich the antenna element 158 provides the lowest output of all antennaelements 158, is identified 102. This can be done e.g. by identifyingthe values of the measurement data, which represent measurements infront of the positions of the antenna elements 158 and comparing thevalues of the measurement data, to determine the first signal path 151,152, 153 with the lowest output.

Further, based on output of the first signal path 151, 152, 153 acorrection factor 126 for the further signal paths 151, 152, 153 iscalculated 103, such that with the applied correction factor 126 theoutput of all signal paths 151, 152, 153 is equal within a predeterminedacceptance interval.

Calculating 103 can comprise dividing the lowest output value, whichrepresents the first signal path 151, 152, 153, by the output value,which represents a respective other one of the signal paths 151, 152,153.

The correction factors 126 can be provided as gain factors for theamplifiers of the respective signal paths 151, 152, 153. The correctionfactors 126 can e.g. be provided as digital values to a transceiver 154or any signal-processing unit of the RF device 150, which drives thesignal paths 151, 152, 153.

The method can also comprise verifying that all signal paths 151, 152,153 provide an output, which is larger than a predetermined minimumoutput.

FIG. 10 shows a block diagram of a calibration system 200 forcalibrating a radio frequency, RF, device 150 as already explainedregarding FIG. 6. The RF device 150 comprises a plurality of signalpaths 151, 152, 153. Each signal path 151, 152, 153 comprises at leastan amplifier 155, 160 and an antenna element 158.

The calibration system 200 comprises a signal generator 201 for drivingthe signal paths 151, 152, 153 with a predetermined test signal 202. Thetest signal 202 can e.g. comprise a radio frequency, RF, signal, whichcan be provided to internal connectors of the RF device 150 for drivingthe signal paths 151, 152, 153 with the RF signal. This serves fortesting the transmit signal chains.

The RF signal can also be provided to probes 203, 204 of the calibrationsystem 200, e.g. antennas, for transmitting the test signal 202 to therespective antenna elements 158 of the respective signal paths 151, 152,153 for testing the receive signal chains.

If the transmit and the receive signal chains are to be tested at thesame time, the signal generator 201 can provide a first sub-signal ofthe RF signal to the probes 203, 204 for transmitting the test signal202 to the respective antenna elements of the respective signal paths151, 152, 153. The signal generator 201 can further provide a secondsub-signal of the RF signal to the RF device 154 for driving the signalpaths 151, 152, 153 with the RF signal.

As an alternative, the signal generator 201 can generate the test signal202, or at least the second sub-signal comprising a digital commandsignal for a transceiver 154 of the RF device 150, which commands thetransceiver 154 to drive the single signal paths 151, 152, 153 with aradio frequency, RF, signal of a predetermined frequency.

The calibration system 200 further comprises at least two probes 203,204 for measuring the output of the signal paths 151, 152, 153 inreaction to the test signal 202. If the probes 203, 204 are used fortransmitting a RF signal to the RF device 150, the output of the signalpaths can be provided by the transceiver 154 to the calibration system200.

The calibration system 200 can also comprise a correction factorcalculator 205 for calculating respective correction factors 206 basedon differences in at least one characteristic of the measured outputs ofthe signal paths 151, 152, 153. The characteristics of the measuredoutputs can comprise a phase and/or an amplitude and/or a frequencyand/or a timing of the outputs. That means that the correction factor206 can be calculated based on a difference in phase of the measuredoutputs, based on a difference in amplitude of the measured outputs,and/or based on a difference in frequency and/or timing of the measuredoutputs.

The correction factor calculator 205 can provide the correction factors206 directly to elements of the respective signal paths 151, 152, 153.As an alternative, the correction factor calculator 205 can provide thecorrection factors 206 as digital values to a signal-processing unit,e.g. the transceiver 154, of the RF device 150, which drives the signalpaths 151, 152, 153.

FIG. 11 shows a block diagram of another embodiment of a calibrationsystem 220, which is based on the calibration system 200 of FIG. 10.Similar elements are provided with similar reference signs as in FIG. 10but increased by 20.

In FIG. 11 the probes 223, 224 are spaced apart from each other by apredetermined distance which defines an exclusion zone 228, 229. Thepredetermined distance can e.g. define a distance between the aperturesof two probes 223, 224 or antennas 223, 224. The predetermined distancecan especially be at least 0.4 times the wavelength of the test signal222.

FIG. 12 shows flow diagram of a calibration method for calibrating aradio frequency, RF, device 150 comprising a plurality of signal paths151, 152, 153, each signal path 151, 152, 153 comprising at least anamplifier 155 and an antenna element 158.

The method comprises driving S201 the signal paths 151, 152, 153 with apredetermined test signal 202. The test signal 202 can e.g. comprise aradio frequency, RF, signal. The RF signal can e.g. be provided tointernal connectors of the RF device 150 for driving the signal paths151, 152, 153 with the RF signal. The RF signal can also be provided tothe probes 203, 204 for transmitting the test signal 202 to therespective antenna elements 158 of the respective signal paths 151, 152,153.

Further, a first sub-signal of the RF signal can be provided to theprobes 203, 204 for transmitting the test signal 202 to the respectiveantenna elements 158 of the respective signal paths 151, 152, 153. Asecond sub-signal of the RF signal can be provided to the RF device 150for driving the signal paths 151, 152, 153 with the RF signal.

The test signal 202 can also be generated as a digital command signalfor a digital signal-processing unit, e.g. a transceiver 154, of the RFdevice 150, which commands the transceiver 154 to drive the singlesignal paths 151, 152, 153 with a radio frequency, RF, signal of apredetermined frequency.

Then the output of the signal paths 151, 152, 153 is measured S202 inreaction to the test signal 202 with at least two probes 203, 204.

The probes 203, 204 can be spaced apart from each other by apredetermined distance, which defines an exclusion zone. Thepredetermined distance can be based on a distance between the aperturesof the two probes 203, 204 and comprise at least 0.4 times thewavelength of the test signal 202.

Respective correction factors 206, 226 are then calculated S203 based ondifferences in at least one characteristic of the measured outputs ofthe signal paths 151, 152, 153.

The characteristics of the measured outputs can comprise a phase, anamplitude, a frequency and/or a timing of the outputs. The respectivecorrection factors 206, 226 can therefore be calculated based on adifference in phase of the measured outputs, based on a difference inamplitude of the measured outputs, and/or based on a difference infrequency and/or timing of the measured outputs.

The calculated correction factors 206, 226 can then be provided directlyto elements of the respective signal paths 151, 152, 153. This meansthat the correction factors 206, 226 directly influence the elements ofthe respective signal paths 151, 152, 153. For example, parameters of aphase shifter 156 and/or an amplifier 155 can be set accordingly.

The calculated correction factors 206, 226 can e.g. be provided asdigital values to a signal-processing unit, e.g. transceiver 154 of theRF device 150, which drives the signal paths 151, 152, 153. Thetransceiver can then pre-process the respective driving signalsaccording to the correction factors 206, 226 or set the parameters inthe respective elements.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations exist. Itshould be appreciated that the exemplary embodiment or exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration in any way. Rather, the foregoingsummary and detailed description will provide those skilled in the artwith a convenient road map for implementing at least one exemplaryembodiment, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope as set forth in the appendedclaims and their legal equivalents. Generally, this application isintended to cover any adaptations or variations of the specificembodiments discussed herein.

In the foregoing detailed description, various features are groupedtogether in one or more examples or examples for the purpose ofstreamlining the disclosure. It is understood that the above descriptionis intended to be illustrative, and not restrictive. It is intended tocover all alternatives, modifications and equivalents as may be includedwithin the scope of the invention. Many other examples will be apparentto one skilled in the art upon reviewing the above specification.

Specific nomenclature used in the foregoing specification is used toprovide a thorough understanding of the invention. However, it will beapparent to one skilled in the art in light of the specificationprovided herein that the specific details are not required in order topractice the invention. Thus, the foregoing descriptions of specificembodiments of the present invention are presented for purposes ofillustration and description. They are not intended to be exhaustive orto limit the invention to the precise forms disclosed; obviously manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. Throughout the specification,the terms “including” and “in which” are used as the plain-Englishequivalents of the respective terms “comprising” and “wherein,”respectively. Moreover, the terms “first,” “second,” and “third,” etc.,are used merely as labels, and are not intended to impose numericalrequirements on or to establish a certain ranking of importance of theirobjects.

The invention claimed is:
 1. A calibration system for calibrating anantenna array comprising a plurality of antenna elements, the systemcomprising: a signal generator for generating a predetermined testsignal and, the signal generator being configured to provide the testsignal to the antenna elements, wherein the signal generator isconfigured to drive single antenna elements of the plurality of antennaelements in a pattern to mitigate mutual influence between neighboringantenna elements, a number of probes for measuring at least one physicalparameter, which is influenced by emissions or receptions of the testsignal by the antenna elements, and providing respective measurementsignals, and a position determination unit for determining based on themeasurement signals the positions of the antenna elements as calibratedpositions.
 2. The calibration system of claim 1, further comprising: amotion actuator for moving relatively to each other the probes and theantenna array, and a recorder for recording the measurement signalstogether with the respective positions of the probes.
 3. The calibrationsystem of claim 2, wherein the position determination unit identifiesthe positions of the antenna elements based on maxima of the measurementsignals and the positions of the respective maxima, or based onpositions of the shifts, at which 1D or 2D auto-correlations of amagnitude pattern of the measured signals provide maxima, or based on atwo dimensional Fourier series or Fourier Transform together with ameasurement heatmap of the measured signals.
 4. The calibration systemof claim 2, wherein the motion actuator moves relatively to each otherthe probes and the antenna array such that the probes move in a firstplane that is parallel to a second plane, in which the antenna elementslie.
 5. The calibration system of claim 4, wherein the motion actuatormoves relatively to each other the probes and the antenna array suchthat the probes move in a plurality of third planes that are parallel tothe first plane and each distanced apart from the prior plane by apredetermined distance.
 6. The calibration system of claim 1, whereinthe motion actuator moves the probes to estimated start positions, whichare estimated to lie in front of respective antenna elements and furthermoves the probes in a pre-defined zone until a maximum measurement valueis identified.
 7. The calibration system of claim 1, wherein the signalgenerator provides the test signal to all antenna elements at the sametime.
 8. The calibration system of claim 1, wherein the signal generatorconsecutively provides the test signal to the antenna elements one byone, or provides the test signal to pairs of the antenna elements or toat least three of the antenna elements at the same time.
 9. Thecalibration system of claim 1, wherein the signal generator generatesthe test signal comprising a radio frequency, RF, signal of apredetermined frequency.
 10. The calibration system of claim 1, whereinthe signal generator generates the test signal comprising a digitalcommand signal for a transceiver of the antenna array, which commandsthe transceiver to drive the single antenna elements with a radiofrequency, RF, signal of a predetermined frequency.
 11. The calibrationsystem of claim 1, wherein the probes comprise a measurement element formeasuring the value of the physical parameter or electromagnetic signalsor wherein the probes comprise a transmitting element for transmittingthe predetermined test signal or electromagnetic signals.
 12. Thecalibration system of claim 11, wherein the measurement elementcomprises an antenna.
 13. The calibration system of claim 12, whereinthe antenna is adapted to the frequency of the test signal.
 14. Thecalibration system of claim 1, wherein the measurement signal comprisesa voltage or a current or a power.
 15. The calibration system of claim1, wherein the physical parameter comprises an electric field or amagnetic field or an electromagnetic field.
 16. A calibration method forcalibrating an antenna array comprising a plurality of antenna elements,the method comprising: generating a predetermined test signal andproviding the test signal to the antenna elements, comprising drivingsingle antenna elements of the plurality of antenna elements in apattern to mitigate mutual influence between neighboring antennaelements, measuring at least one physical parameter with a number ofprobes, which is influenced by emissions or receptions of the testsignal by the antenna elements, and providing respective measurementsignals, and determining based on the measurement signals the positionsof the antenna elements as calibrated positions.
 17. The calibrationmethod of claim 16, further comprising: moving relatively to each otherthe probes and the antenna array, and recording the measurement signalstogether with the respective positions of the probes.
 18. Thecalibration method of claim 17, wherein determining the positionscomprises identifying the positions of the antenna elements based onmaxima of the measurement signals and the positions of the respectivemaxima, or based on positions of the shifts, at which 1D or 2Dauto-correlations of a magnitude pattern of the measured signals providemaxima, or based on a two dimensional Fourier series or FourierTransform together with a measurement heatmap of the measured signals.19. The calibration method of claim 17, wherein moving comprises movingrelatively to each other the probes and the antenna array such that theprobes move in a first plane that is parallel to a second plane, inwhich the antenna elements lie.
 20. The calibration method of claim 19,wherein moving comprises moving relatively to each other the probes andthe antenna array such that the probes move in a plurality of thirdplanes that are parallel to the first plane and each distanced apartfrom the prior plane by a predetermined distance.
 21. The calibrationmethod of claim 16, wherein moving comprises moving the probes toestimated start positions, which are estimated to lie in front ofrespective antenna elements and further moving the probes in apre-defined zone until a maximum measurement value is identified. 22.The calibration method of claim 16, wherein providing the test signalcomprises providing the test signal to all antenna elements at the sametime.
 23. The calibration method of claim 16, wherein providing the testsignal comprises consecutively providing the test signal to the antennaelements one by one, or provides the test signal to pairs of the antennaelements or to at least three of the antenna elements at the same time.24. The calibration method of claim 16, wherein providing the testsignal comprises generating the test signal comprising a radiofrequency, RF, signal of a predetermined frequency.
 25. The calibrationmethod of claim 16, wherein providing the test signal comprisesgenerating the test signal comprising a digital command signal for atransceiver of the antenna array, which commands the transceiver todrive the single antenna elements with a radio frequency, RF, signal ofa predetermined frequency.
 26. The calibration method of claim 16,wherein the probes are provided with a measurement element for measuringthe value of the physical parameter or electromagnetic signals orwherein the probes are provided with a transmitting element fortransmitting the predetermined test signal or electromagnetic signals.27. The calibration method of claim 26, wherein the measurement elementcomprises an antenna.
 28. The calibration method of claim 27, whereinthe antenna is adapted to the frequency of the test signal.
 29. Thecalibration method of claim 16, wherein the measurement signal comprisesa voltage or a current or a power or a phase of the measured physicalparameter.
 30. The calibration method of claim 16, wherein the physicalparameter comprises an electric field or a magnetic field or anelectromagnetic field.